<!DOCTYPE html>
<html>
  <style>
    input { color: green; }
    input.notdefault:default { color: red; }
    input.default:not(:default) { color: red; }
  </style>
  <body>
    <form id="one"></form>
    <input type="submit" form="one" class="default" id="test">
    <form id="two">
      <input type="submit" class="notdefault">
    </form>
    <script>
      document.body.offsetWidth;
      document.getElementById("test").setAttribute("form", "two");
    </script>
  </body>
</html>
